1. Technical Field
The present teaching relates to electrical circuits. Particularly, the present teaching relates to analog-to-digital converters.
2. Discussion of Technical Background
Analog-to-digital converters (ADCs) are used for a wide range of applications, including, but not limited to, sensor interfaces, industrial applications, consumer applications, and communications. Various circuits and techniques have been developed for analog-to-digital (A/D) conversion targeting various applications and their varying requirements in terms of speed, resolution, noise, power consumption, and other performance related parameters.
An analog-to-digital converter (ADC) may be configured to provide a numerical representation of a differential signal, such as for example a voltage difference provided by a strain-gauge sensor circuit. The voltage difference may provide information about a strain parameter (deflection, torque, etc.) for an axle of a machine, and a numerical representation of the voltage difference may be used as an input for a digital control system regulating power delivered to an electrical motor coupled to the axle. If the numerical representation is a false indication of the actual strain parameter, the digital control system may cause the electrical motor to damage the axle and/or structures connected to the axle. It is desirable that a digital control system be able to detect that it may be receiving false indications, so that measures can be taken to prevent damage and injuries.
FIG. 1 shows a prior-art configuration of a digital control system 100 observing a signal from a strain-gauge sensor 101. The strain-gauge sensor 101 provides a voltage difference that is amplified by an instrumentation-amplifier circuit 102. Instrumentation-amplifier circuit 102 amplifies the voltage difference by a predetermined amplification factor so that a full-scale stimuli of the strain-gauge sensor 101 corresponds to a full-scale range of a digital representation provided by ADC circuit 103 to a digital system 104. An overall gain factor is nominally independent of a reference voltage VREF provided to strain-gauge sensor 101 and to ADC circuit 103. A nominal operation of ADC circuit 103 may be characterized by D=(V1−V2)/VREF, where D is a numerical value designated by a digital code with N bits. V1 is a voltage (relative to a reference node/terminal, which may be referred to as ground) applied to a first input terminal of ADC circuit 103. V2 is a voltage applied to a second input terminal of ADC circuit 103. ADC circuit 103 may provide 16-bit codes to digital system 104 to designate numerical values in a numerical full-scale range having a lower limit (−32768/32768) and an upper limit 32767/32768. The lower limit may nominally correspond to a stimuli of strain-gauge sensor 101 for which V1˜0V and V2˜VREF. The upper limit may nominally correspond to a stimuli of strain-gauge sensor 101 for which V1˜VREF and V2˜0V. Strain-gauge sensor 101 and instrumentation-amplifier circuit 102 may be designed such that, during normal operation, a change of voltage V1 will correspond to an equal-but-opposite change of voltage V2. For example, normal operation may be characterized by a nominal constraint V1+V2˜VREF. Any substantial deviation from the nominal constraint indicates that D may be a false representation of the stimuli of strain-gauge sensor 101. Prior-art ADC circuit 103 provides only one numerical parameter D to characterize the two analog input voltages (V1,V2), and digital system 104 is not able to detect that D=(V1−V2)/VREF=0 may be a false reading when, for example, V1=V2=VREF/4.
What is needed is an analog-to-digital converter circuit and method to provide a plurality of parameters for a two-input analog signal so that relatively more robust, flexible and fault-tolerant systems involving analog-to-digital conversion can be implemented.